The concept of Stack Exchange is to connect subject matter experts of various subjects with people who might have questions about the subjects they are expert in.
Nobody's going to hang about to answer questions on a site entitled Miscellaneous because it won't have a scope that really makes sense. We need to ensure that experts are concentrated in areas where there are questions for them to answer, otherwise nobody will get useful answers.
Additionally, if we get a question that's on topic elsewhere is it automatically off-topic on Miscellaneous? Or if we launch a new site that covers one of the topics that previously fell under Miscellaneous does that change the scope of Miscellaneous? We'll forever be arguing about whether questions should be migrated from Miscellaneous or not. Miscellaneous would likely end up as a dumping ground for poor quality questions that no other site would touch.
In fact in its early days Stack Exchange did create such a site. It was called non-programming and they said it was for everything that Stack Overflow was not. It failed. It never gained any critical mass of answerers who were prepared to use it. People asked their questions, got nowhere and never used the site again.